An exact reconstruction of the shape of an existing cavity, for example an opening, cave or the ear canal, is needed for various applications to be able to rebuild an exact copy of the cavity,
To provide an optimal seat and create also optimal acoustic conditions of elements of hearing devices to be placed into the ear canal, as for example speakers or hearing aids, these elements need to be adapted to the geometry of the ear canal. This geometry is predefined by the physiognomy and is always different for each user.
A known possibility for the customization of these elements consists in offering different cases of the elements for certain basic forms of the ear canal or in equipping the outer wall of the cases with an elastic material which permits a certain adaptation to different designs. This method is, however, only suitable for very small elements and offers neither an optimal seat nor optimal acoustic conditions. The pressure exerted by the elastic surface of the elements onto the ear canal moreover is found unpleasant and can cause even pains.
To get an exact reproduction of the ear canal, especially the outer ear canal, three-dimensional imprints have commonly been taken, from which moulds could be formed for the production of elements of hearing devices with the desired shape. Therefore, Silicone is commonly injected into the ear canal for preparing the imprint. To receive a reproduction as exact as possible of the ear canal, this must be cleaned essentially, i.e. be free of contaminates. As a rule, the whole process lasts between three and five minutes. For the protection of the eardrum a stopper must be brought deeply into the ear canal, and must be removed again after drying the silicone. This process altogether will be regarded by the person affected as rather unpleasant and disturbing. The forming of the case of the hearing device element using the imprint requires between 1.5 and 2 hours, after which frequently further customizations are necessary after putting the case into the ear canal until the definite fit is found.
Broader possibilities for the recording of the geometry of the surface form of cavities lie in the use of ultrasonic apparatuses or X rays. At the first method, the recording of the form of the human ear canal implies relatively high frequencies and it would be necessary that the surroundings of the ear must be freed from growth of hair. Furthermore, the deep inner portions of the ear canal may be covered by other portions of the ear canal and thus preventing the recording of those inner portions. Further problems exists with the exact transmission of the ultrasound either by the body or over the air, to bring about the necessity for separate transmission means (for example a gel), which means on the one hand a great effort and on the other hand can show unpleasant side effects.
The second method, the use of X rays, needs a very expensive equipment and is only conditionally suitable for the desired purpose.
As a rule, the use of a mechanical scanning device will fail if the cavity shows passages full of corners and isn""t feasible if the cavity to be determined falls below a certain size as this is the case, for example for the ear canal.
An object of the present invention is to provide a method for the fast and reliable reconstruction and saving of the geometry of a cavity.
A further object of the present invention is to provide such a method for the fast and reliable reconstruction of at least the outer portion of the ear canal.
In a preferred embodiment of the invention, for the recognition of the position data, such as topographical data, in a three-dimensional coordination system, a probe with an optical sensor will be introduced from the outside through the opening of a cavity to be determined, such as the human ear canal. The optical sensor is thereby producing video signals or video images, which will be transmitted to a computer device. The computer will then compute by means of known algorithms from the set of video images received by the optical sensor the positioning data of the inner wall of the cavity. Preferably the iterative factorization methods by Poelman and Kanade (Conrad Poelman and Takeo Kanade, xe2x80x9cA Paraperspective Factorization Method for Shape and Motion Recoveryxe2x80x9d, Technical report CMU-93-219, Carnegie Mellon University, Pittsburgh) and Han and Kanade (Mei Han and Takeo Kanade, xe2x80x9cPerspective Factorization Methods for Euclidean Reconstructionxe2x80x9d, Technical report CMU-RI-TR-99-22, Carnegie Mellon University, Pittsburgh) will be used to calculate the three-dimensional coordinates of the surface of the cavity. These coordinates now may be directly used to produce a mould by means of numeric controlled tools with the exact shape of the inner wall of the cavity.
As an advantage, the unpleasant injection of Silicon into the ear canal may be prevented as well as the use of an additional stopper to protect the eardrum.
Even more advantageous is the fact, that the entire method is carried out within a very short time with no need of any preparation work around or in the cavity. The only condition to be observed is a clean cavity, to produce the exact shape of the inner wall of the cavity. A further advantage of the present invention is the possibility of performing the measuring of the shape without having an direct and defined reference to the vicinity of the cavity itself, that is the catheter needs not to be calibrated in respect to the cavity and may be moved without guiding aids.
In a further preferred embodiment contrasting points with a diameter of about 50 xcexcm will be applied onto the surface of the cavity. These contrasting points enable an optimal recognition rate by the optical sensor and the algorithm used to determine the positioning data. The contrasting points may preferably be applied in form of ink droplets which may be sprayed onto the surface of the cavity. The use of ink or fluorescent ink provides on one hand a good contrast in relation to the cavity wall itself for reliable optical detection and on the other hand is easy to handle, that is easy to be sprayed and afterwards removed.
The contrasting points may be applied directly onto the surface of the cavity, which therefore only needs to be clean. The cleaning of the ear canal of example may be performed by a simple wash up of the ear canal.
In a further embodiment of the present invention, the contrasting points are represented by particles, preferably small particles which can be fluorescent.
In a further embodiment of the present invention, the contrasting points may be applied in advance onto the surface of a separate thin, elastically body, such as a balloon, which may then be introduced or positioned in to the cavity and pressed close to the surface of the cavity before the insertion of the catheter. This may be performed by applying an overpressure to the inside of this thin body or balloon or by sucking out the air between this thin body and the surface of the cavity. The contrasting points may thereby have already been applied onto the surface of the thin body, either onto the inside or outside of the surface, or may be sprayed or applied after the insertion and inflation of the thin body.
In a further preferred embodiment the contrasting points are transferred by a film onto the surface of the cavity. This method allows the preparation of such films with specific arrangement of the contrasting points with respect of size, shape and distance.
Furthermore, the contrasting points in form of particles may be applied to the surface of the cavity by means of a liquid coat or may be transferred by a foam containing such particles.
In a further preferred embodiment the optical sensor is arranged as flexible or rigid probe. The use of a flexible probe allows the recognition of the surface of cavities with strongly crooked passages. The imaging device may either be arranged at the proximal end of the probe, with glass fibers or lenses for the transmission of the image, or directly at the distal end of the probe. The imaging device is preferably a CCD, CMOS or analog camera device. For the use in small cavities, such as ear canals, the probe diameter is about 2 mm or less, with a focal length of about 2 mm and a resolution of about 50 xcexcm. These parameters allows the recognition of the exact shape of the cavity in a sufficient resolution.
The probe will be inserted into the cavity by the outlet of the cavity, thereby performing a linear forward movement and preferably at the same time a rotational movement around its longitudinal axis thus allowing to cover the entire surface of the cavity, at least in the area to be determined.
The video signals will be preferably transmitted to a computer device, which will perform algorithms to transform those video signals into three-dimensional coordinates of the surface of the cavity.
In a preferred embodiment the video signals from a optical device with telecentric projection are directly treated by using a factorization method, preferably the factorization methods by Poelman and Kanade (Conrad Poelman and Takeo Kanade, xe2x80x9cA Paraperspective Factorization Method for Shape and Motion Recoveryxe2x80x9d, Technical report CMU-93-219, Carnegie Mellon University, Pittsburgh) and Han and Kanade (Mei Han and Takeo Kanade, xe2x80x9cPerspective Factorization Methods for Euclidean Reconstructionxe2x80x9d, Technical report CMU-RI-TR-99-22, Carnegie Mellon University, Pittsburgh) to calculate the three-dimensional coordinates of the surface of the cavity, especially the coordinates of the contrasting points of the surface of the cavity.
Furthermore a nonlinear optimization method may be used to calculate the three-dimensional coordinates of the surface of the cavity, especially the coordinates of the contrasting points of the surface of the cavity.
Furthermore, any of the above mentioned reconstruction methods may be applied to subsets of the data and the reconstructed three-dimensional coordinates of these subsets can be combined using methods of 3D geometry to give a 3D reconstruction of all of the cavity surface.
Further a progressive reconstructions may take place, by using the preceding subset of data as an aid for the reconstruction of a succeeding subset of data.
The basis of the above reconstruction algorithm lies in the feature recognition, i.e. the identification and location of point features in each video image generated by the probe. Apart form the use of artificial contrasting points, natural features such as surface colors, blood vessels, surface profile features, e.g. wrinkles, or features attached to the surface, e.g. hairs, may be sufficient for the algorithms as described above.
The present method is not only suited to be used in the medical field, such as the reconstruction of the ear canal for the fabrication of hearing devices or hearing aids or the three-dimensional reconstruction of shapes from images acquired by an optical sensor such as en endoscope, but also for the reconstruction of underground caves, internal spaces within machinery or voids within collapsed structures.